The object management arrangement may include information passing between the interrogator and the electronically coded tags, which respond by issuing a reply signal that is detected by the interrogator, decoded and consequently supplied to other apparatus in the sorting, controlling or auditing process. The objects to which the tags are attached may be animate or inanimate. In some variants of the system the frequency of the interrogating or powering field may range from LF to UHF or Microwave.
An electromagnetic source is required to create a field which may energise a tag's circuitry and/or illuminate an antenna associated with a tag for backscatter, depending on whether the tag is passive or active, eg. battery assisted.
To couple to all tags in a randomly oriented collection, when either a collection of tags or the field creation structure moves, a flux line must exist which couples to a tag in any orientation. This may be achieved simply by ensuring that multiple, eg. three, electromagnetic sources are used, each with its axis oriented in a different direction, with a most efficient case being three orthogonal directions of a Cartesian co-ordinate system. When two sources or multiple sources are used having only two unique source axes, a randomly oriented tag may not couple to a flux line when moved through the field or when the source structure is simply translated along one direction, and hence may not be read. However, if either the tag or antenna structure is itself rotated during traversal of the tag or translation of the antenna structure, the tag may couple to a flux line. Assuming that traversal and/or rotation allows a coupling flux line to dwell at a required direction for long enough, the tag should complete its reply and be read.
The present invention is related to apparatus disclosed in PCT application 2004/000175 entitled “System for Minimizing Coupling Nulls within an Electromagnetic Field”, the disclosure of which is incorporated herein by cross reference. In the system disclosed in PCT application 2004/000175, the or each tag bearing object is translated and/or rotated relative to an electromagnetic field produced by a reading antenna such that no tag is persistently located in a coupling null relative to the field.
Reliable reading performance is possible with the prior art system under a condition that a tagged object is rotating or is otherwise changing its orientation relative to a reading antenna. However, a disadvantage of the system is that objects that do not rotate or otherwise change their orientation relative to a reading antenna many not be read reliably. Additionally, implementation of the antenna arrangement is relatively complex requiring a multiplexing device and switches that need to be monitored and adjusted by skilled operators. The system is therefore maintenance intensive and is prone to detuning and other undesirable effects.
An aim of the present invention is to provide a system for minimizing coupling nulls between an electromagnetic field associated with a tag reading antenna and a plurality of randomly oriented tags by dynamically changing the orientation of the electromagnetic field relative to a tag or tags as the tag or tags pass through or past the reading antenna.